Patient specific QA for RapidArc

Abstract

Abstract: RapidArc is a volumetric arc therapy which was recently introduced to Ireland by St. Lukes Radiation Oncology Network. RapidArc is characterised by continuous MLC and gantry motion in combination with varying gantry speed, dose rate and MLC speed. This approach leads to significantly shortened treatment times and dose conformity which is comparable to that achieved by static IMRT. Because of the complexity of RapidArc treatment plans, it is vital to implement an efficient and accurate protocol for routine patient specific QA. We decided to use the ArcCHECK phantom for this purpose. ArcCHECK is a commercial diode array designed specifically for rotational measurements. Our ArcCHECK measurements were validated by comparison with Gafchromic film measurements as this is a standard high resolution patient verification method. Specifically, our work involved delivering a number of prostate RapidArc treatment plans to ArcCHECK. Independent validation of ArcCHECK measurements with Gafchromic film, in two orientations was performed for a subset of these plans. Gafchromic film analysis was performed using the ‘triple channel dosimetry’ approach. This is a new method of evaluating radiochromic film dosimetry data which improves the integrity of the dose information in the film by removing disturbances in scanned images. As a result of following this method, a high level dosimetric accuracy was evident in our film measurements. Dose distributions were analysed using gamma criteria of 3% dose and 3 mm distance to agreement. A pass rate of at least 95% was achieved using data from ArcCHECK and Gafchromic film respectively. This work combines the construction of an efficient protocol for using ArcCHECK for patient specific QA of RapidArc treatment plans with an implementation of the ‘triple channel dosimetry’ approach for Gafchromic film analysis. Thus we have formulated a protocol for patient specific QA of RapidArc treatment plans and incorporated Gafchromic film as a suitable backup system.